To the left is the matrix of cell configurations that lead to the central cell being turned on (highlighted) or turned off (dim) in the next step.
The determination of a cell's on/off state in the next step is made by consulting the rule that corresponds to the states of each of the six neighbors (x+, x-, y+, y-, z+, z-) and the present state of the cell itself.
For example, the rule to the right says that if a cell is on and the state above it (y+) is on, the cell will turn off in the next step.
If the rule is highlighted , the cell will be "on" in the next step.